Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric data tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 comprising a number of servo sectors 40-4N recorded around the circumference of the disk which define a plurality of servo tracks 6. Each servo sector 4i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 4i may further comprise groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
The geometry of the write element of the head may impact the ability to reliably recover written data, particularly toward the outer and inner diameter of the disk where the head skew angle increases. For example, in disk drives employing perpendicular magnetic recording, the width of the magnetic footprint generated by the write element may increase due to the head skew angle. This is illustrated in FIG. 2A which shows that the total write width (TW) increases beyond the width (WW0) of the write element due to the head skew angle αs exceeding the bevel angle αod. In order to reduce the amount of adjacent track erasure, the data tracks per inch (TPI) is typically adjusted across the radius of the disk as shown in FIG. 2B so that the track width of the data tracks is increased toward the outer and inner diameter of the disk. However, selecting the optimal TPI that minimizes adjacent track erasure while achieving a desirably high radial density depends on the actual geometry of the write element.
There is, therefore, a need to estimate the head geometry in a disk drive, and to use the estimated head geometry to select a TPI across the radius of the disk.